1. Field of the Invention
The present invention relates to thermal printing, and more particularly, to an apparatus for providing back tension on a print media web as it is transported through a thermal printer.
2. Description of Related Art
In the field of bar code symbology, vertical bars of varying thicknesses and spacing are used to convey information, such as an identification of the object to which the bar code is affixed. Bar codes are often printed onto a print media comprising individual paper substrate labels having an adhesive backing layer that enables the labels to be affixed to objects to be identified. To read the bar code, the bar and space elements of the bar code may be scanned by a moving light source, such as an articulating laser beam. Alternatively, the bar and space elements may be imaged in one or two-dimensions by a photosensitive imaging element, such as a charge coupled device. Since the bar and space elements have differing light reflective characteristics, the information contained in the bar code can be read by interpreting the reflected light or image pattern from the bar code. In order to accurately read the bar code, it is thus essential that the bar code be printed in a high quality manner, without any streaking, blurring or misregistration of the bar code. At the same time, it is essential that the adhesive backing layer of the labels not be damaged by heat generated during the printing process.
In view of these demanding printing requirements, bar codes are often printed using direct thermal or thermal transfer printing techniques. In direct thermal printing, the print media is impregnated with a thermally sensitive chemical that is reactive upon exposure to heat for a period of time. Thermal transfer printing requires an ink ribbon that is selectively heated to transfer ink to the print media. These printing techniques are referred to collectively herein as thermal printing.
To print the bar code, the print media is drawn between a platen and a thermal print head of the printer. The thermal print head has linearly disposed printing elements that extend across a width dimension of the print media. The printing elements are individually activated in accordance with instructions from a printer controller. As each printing element is activated, the thermally active chemical of the ribbon activates at the location of the particular printing element to transfer ink to the printed area of the print media. The print media is continuously drawn through the region between the platen and the thermal print head, and in so doing, the bar code is printed onto the print media as it passes through the region. Other images, such as text, characters or graphics, can be printed in the same manner. The thermal printer includes a mechanism for transporting the print media from a supply hub to the print region. Such printers typically include a media post or hub onto which is mounted a supply roll of the print media material. The media post is substantially smaller in cross-section than the core of the media supply roll so that the post can accommodate media supply rolls of various sizes. As the media supply roll is rotated under take-up tension applied by the transporting mechanism, a web of the print media is paid out and transported past the thermal print head.
The movement of the print media must be precisely coordinated with the operation of the thermal print head in order to ensure accurate registration of the printed information to the print media. Top-of-form registration refers to the alignment of the printed information to the beginning or leading edge of a print media label. A top-of-form registration error occurs when the printing begins either too soon or too late. Sensing circuits are often utilized to detect the gap between adjacent labels in order to accurately coordinate the printing to the leading edge. Lateral registration refers to the alignment of the printed information to the left and right side edges of the label, and a lateral registration error occurs when the print media supply roll is not properly stabilized with respect to the media post. Print media labels often include pre-printed material with designated spaces for printing bar codes and other information. Thus, registration errors can result in overlapping between the pre-printed material and the newly printed information, causing the newly printed information to become obscured. In some cases, a lateral registration error could cause the bar code symbol or text to run off the edge of the label, rendering the bar code unreadable.
Both types of print registration error conditions can be adversely effected by insufficient back tension applied to the print media web. Initially, the weight of the media supply roll is sufficient to maintain a positive connection between the media supply roll and the media post to prevent the web from paying out at an uneven rate or wandering laterally on the media post. As the media supply roll is paid out, however, its size and weight decreases until it no longer provides sufficient back tension to the web. Thereafter, the supply roll starts lifting off of the media post and shifting position laterally along the media post. In turn, this causes the pay out rate of the print media to fluctuate which ultimately results in undesirable degradation of both lateral and top-of-form registration.
Accordingly, it would be desirable to provide a mechanism for a thermal transfer printer that would provide sufficient back tension on the rotating media supply roll in order to maintain a consistent transport rate of the media web as it is paid out. At the same time, the mechanism should not overly complicate the printer or substantially increase its production cost.